Laminates for printed circuit boards are formed by stacking a prescribed number of prepregs comprising an electrical insulating resin composition as the matrix, and subjecting them to heat and pressure for integration.
Metal-clad laminates are used for formation of printed circuits by a subtractive process. Metal-clad laminates are fabricated by stacking a metal foil such as copper foil on a surface (either or both surfaces) of a prepreg and subjecting the stack to heat and pressure. As electrical insulating resins there are commonly used thermosetting resins such as phenol resins, epoxy resins, polyimide resins, bismaleimide-triazine resins and the like. Thermoplastic resins such as fluorine resins or polyphenylene ether resins are also sometimes used.
On the other hand, with the increasing popularity of data terminal devices such as personal computers and cellular phones there is a trend toward miniaturization and high-densification of the printed circuit boards mounted therein. The mounting methods are advancing from pin-insertion types to surface-mounted types, and also to area array types of which BGA (ball grid arrays) using plastic boards are a typical example. In boards where bare chips such as BGA are directly mounted, connection between the chips and boards is commonly achieved by wire bonding using thermosonic bonding. In such cases, the board on which the bare chip is mounted is exposed to high temperatures of 150° C. and above, and therefore the electrical insulating resin must have some degree of heat resistance.
As lead-free solders become more common for environmental reasons, the melting points of solders are increasing as a result. Thus, even higher heat resistance is demanded for boards. Demand for halogen-free materials is also increasing, thus precluding the use of bromine-based flame retardants.
Moreover, for printed circuit boards, the “repair properties” allowing mounted chips to be removed are often required. During repair, the boards are subjected to about the same level of heating as during chip mounting, after which further heat treatment is carried out for remounting of chips. Such treatment has often resulted in peeling between the fiber material and resin in conventional insulating resin systems. Thus, boards exhibiting repair properties must also have cycling heat shock-resistant properties at high temperatures.
Prepregs have been proposed that exhibit excellent heat shock resistance, reflow resistance and crack resistance, and improved microwiring formation properties, by impregnating a resin composition comprising a polyamideimide as an essential component into the fiber material (see Japanese Unexamined Patent Publication No. 2003-55486).
Also, with the trend toward greater miniaturization and higher performance of electronic devices, it is becoming necessary to house part-mounted printed circuit boards in increasingly limited spaces. This is accomplished by methods of arranging multiple printed circuit boards in stacks and connecting them alternately with wire harnesses or flexible wiring boards. There are also used rigid-flex boards which are layered combinations of polyimide-based flexible boards and conventional rigid boards.